1. Description of the Prior Art
As is well known to those skilled in the art, conventional ethylene manufacturing facilities utilize cooling water, which is generally available at a temperature of about 80.degree. F., to cool various equipment and process streams during various stages of ethylene production. Cooling water is utilized as the cooling medium in the primary frationation and cooling zone to cool the hot quench water, in the compression zone to condense the steam used to drive steam turbines which power cracked gas compressors and to cool the compressor cracked gases. It is also used in the purification zone to cool overhead vapors in the condensers associated with fractionation towers and in the propylene refrigeration system, which is a part of the purification zone, to condense both the propylene vapor which has been compressed and the steam used to drive turbines which power the propylene compressors.
Some of the equipment or apparatus described above develop low levels of heat which is either rejected to cooling water or to the atmosphere. For example, the low-level waste heat which is generated in the turbine exhaust of a steam gas turbine compressor is, under conventional techniques, rejected to either cooling water or the atmosphere. Heretofore, the rejection of available low-level heat generated in the various equipment as explained previously was, from an economical standpoint, not entirely unobjectionable since fuel was relatively inexpensive and readily available. Unfortunately, however, the relatively high cost of today's fuels, coupled with present efforts to conserve energy, now makes it necessary that new procedures be developed so as to minimize the amount of fuel required for an ethylene production system and, if possible, to capture sources of energy such as is present in low-level waste heat for possible use in the ethylene production plant.
Attempts to utilize low-level waste heat in ethylene production have not heretofore been entirely satisfactory due primarily to the fact that the amount and temperature of the heat generated were not sufficient to utilize the heat as a heating source for the equipment, which in some cases requires temperatures sufficient to convert water to steam. As mentioned previously, cooling water has been the cooling medium utilized to cool equipment and process streams. An ammonia absorption refrigeration system has already been proposed as a possible technique for cooling certain equipment in oil refineries and elsewhere such as is shown in Refiner & Natural Gasoline Manufacture, Vol. 20, No. 5, May, 1941, page (146) 56 and U.S. Pat. No. 3,817,050 (issued June 18, 1974). However, no satisfactory system has heretofore been proposed for supplying the energy required for generating the ammonia refrigerant.
It has been found that the low-level waste heat produced during ethylene production can be effectively utilized to generate refrigerant in an ammonia absorption system used for cooling equipment and process streams. Advantageously, no additional outside sources of heat are required to generate the ammonia refrigerant and, since cooling of the process streams is effected at temperatures significantly lower than could be obtained using cooling water, expensive process equipment can be either eliminated or reduced in size and the overall cost of producing ethylene can be significantly reduced.
It is therefore an object of the present invention to provide a cooling method for cooling process streams produced during ethylene manufacture which method utilizes low-level waste heat, e.g., the heat of hot water or low-pressure steam, to generate refrigerant in an ammonia absorption refrigeration system.
Another object is to utilize a refrigeration method in ethylene production which method permits lower operating temperatures and pressures in the product recovery purification zone.
A further object is to provide a refrigeration system which can be readily integrated into the overall ethylene production process to achieve the more economical production of ethylene.
These and other objects will become apparent from the following summary of the invention and description taken in conjunction with the accompanying drawing.